1. Field of the Invention
The present invention relates to a fuel injection device in which air is jetted out to liquid fuel injected through a fuel injection valve to accelerate atomization of the liquid fuel.
2. Description of the Prior Art
Several types of air-assisted fuel injection devices for accelerating atomization of liquid fuel are known. Japanese Utility Model Unexamined Publication No. 54-44821 discloses a device which is provided with a mixing chamber at the mount portion of a fuel injection valve, the mixing chamber being connected to a spring chamber of a pressure adjuster to modify a pressure of supplied fuel depending upon the pressure in the mixing chamber. Japanese Patent Publication No. 55-9555 discloses a device in which an air amount adjusting valve operated responsive to an engine temperature is disposed in an air bypass pipe associated with an intake manifold, the downstream end of the air bypass pipe is open in the vicinity of the air amount adjusting valve, and further the opening at the downstream end of the air bypass pipe has an effective sectional area smaller than that of the air bypass pipe in a portion downstream of the air amount adjusting valve. Additionally, Japanese Patent Publication No. 2-8144 discloses a device in which a modifying coefficient is determined based on a pressure essentially equal to the air pressure in an air introducing chamber, and the valve-opening time of a fuel injection valve is determined by the modifying coefficient to obtain an optimum amount of fuel injected at all times.
Particularly in the case of low-displacement engines or multi-cylinder engines, however, any of the above air-assisted fuel injection devices requires assist air to be reduced in amount or cut off during warm-up, because the amount of assist air bypassing a throttle valve becomes too much after the completion of warm-up and the idling rotational speed is so raised as not to maintain a predetermined rotational speed. But cut-off of the assist air causes a shift from the state that fuel is satisfactorily atomized with the aid of the assist air to the state that fuel is not sufficiently atomized, resulting in an increase of manifold wetting. The manifold wetting is increased particularly in cold weather and the fuel supplied to the engine becomes insufficient. As a result, the air/fuel ratio for combustion is fluctuated to produce a lean air/fuel ratio, giving rise to such a problem that the idling rotational speed is dropped down or disturbed (called rough idling) (see FIG. 11).